The invention relates to a tire having a radial carcass reinforcement which is intended to be fitted on a heavy vehicle, such as a transport vehicle or construction machinery. It relates more particularly to the crown reinforcement of such a tire.
The prior art construction-vehicle tire shown in FIG. 1 comprises, as is known per se, a carcass reinforcement 1 composed of a single ply of inextensible metal cables made of steel, anchored within each bead to a bead wire 2 to form an upturn 10, the end of which is substantially located at the level S0 of the greatest axial width of the carcass reinforcement. Said carcass reinforcement is radially surmounted by a layer 20 and profiled members 21 of rubber mix, then by a crown reinforcement 3 composed firstly of two plies 31 and 32 referred to as working plies and secondly radially of two so-called protective plies 51 and 52. The working plies are formed of inextensible metal cables made of steel, which are parallel to each other within each ply 31, 32 and are crossed from one ply 31 to the next 32, forming angles which may be between 15xc2x0 and 45xc2x0 with the circumferential direction. The axial widths of said working plies are generally between 60% and 80% of the maximum axial width of the carcass reinforcement 1. The protective plies 51, 52 are generally formed of elastic metal cables made of steel, which are parallel to one another within each ply 51, 52 and are crossed from one ply 51 to the next 52, also forming angles which may be between 15xc2x0 and 45xc2x0 with the circumferential direction. The widths of said protective plies 51, 52 are usually less than the width of the widest working ply. Finally, the cables of the radially outer working ply 32 are usually crossed with the cables of the radially inner protective ply 51. The crown reinforcement is itself surmounted by a tread 4 which is joined to the two beads 7 by the two sidewalls 6.
As is known, the crown reinforcements of radial tires, and more particularly large-dimension tires, are subjected to great deformation, which causes longitudinal and transverse shearing stresses between the edges of two crossed plies (the longitudinal shearing is greater than the transverse shearing when the cables of crossed plies form small angles with the circumferential direction), at the same time as a delamination stress, a radial stress which tends to separate the edges of the two plies radially. Said stresses are due first and foremost to the inflation pressure of the tire, which means that the so-called belting pressure between the carcass reinforcement and the crown reinforcement tends to cause the circumferential expansion of said crown reinforcement; next they are due to the load borne by the tire when travelling, with a contact surface being produced between the ground and the tire; then finally to the drifting of the tire when travelling. Said shearing stresses generate fissures in the rubber mix adjoining the end of the shortest ply, which fissures spread within said mix and adversely affect the life of a crown reinforcement, and therefore of the tire.
A clear improvement in the endurance was obtained by using in the crown reinforcement at least one protective crown ply having an axial width greater than the width of the axially widest working ply.
Another solution, as noted in French Patent 2 421 742, consists in more favourably distributing the stresses causing separation between working crown plies, following the drifting of the tire, by multiplying the number of working plies, for example by using four working plies of reinforcement elements crossed from one ply to the next, forming angles which may be between 15xc2x0 and 35xc2x0 with the circumferential direction and by distributing the reinforcement elements used in the conventional two working plies on the four working plies such that the latter each have the same thickness and the same rigidity in extension measured perpendicular to the reinforcement elements of the ply.
Multiplication of the working plies is not without its drawbacks, particularly at the center of the reinforcement where the number of plies has a very great influence on the flexural strength of the crown of the tire. The invention proposes increasing the resistance to separation between working plies of a crown reinforcement having a widened protective ply plies and thus to improve the endurance of such a crown reinforcement for a tire for construction machinery, without increasing the number of working plies at the center of the reinforcement.
The tire according to the invention, comprising at least one radial carcass reinforcement anchored within each bead to at least one bead wire, forming an upturn, said reinforcement being surmounted radially to the outside by a crown reinforcement formed of at least three so-called working plies formed of inextensible metal reinforcement elements which are parallel to each other within each ply and are crossed from one ply to the next, forming angles xcex1, xcex1xe2x80x2 of between 15xc2x0 and 35xc2x0 with the circumferential direction, is characterized in that at least one half-ply, formed of inextensible metal reinforcement elements and of width Lxe2x80x2, on either side of the equatorial plane, is arranged radially between the edges of at least two radially adjacent working plies, the axially outer and inner ends of said half-ply being located, respectively, axially to the outside of the end of the widest working ply and axially to the inside of the end of the least wide working ply, at distances at least equal to the amount Lxe2x80x2/5, the reinforcement elements of said half-ply forming with the circumferential direction an angle xcex2, which is firstly greater in absolute value than 25xc2x0, and secondly is greater in absolute value than the largest angle formed by the elements of the two working plies by an amount between 5xc2x0 and 15xc2x0.
Preferably, a half-ply is arranged between the two working plies closest to the carcass reinforcement. Whether there be one or two half-plies, the reinforcement elements of each half-ply are advantageously crossed with the reinforcement elements of the working ply radially to the inside of said half-ply and closest radially to the carcass reinforcement.
The widths of the working plies of the crown reinforcement, which are generally unequal, are such that the working ply radially to the inside of each half-ply is less wide than the ply radially to the outside of said half-ply.
In the case of the presence, on either side of the equatorial plane, of two half-plies arranged between the edges of two radially adjacent working plies of the working reinforcement of three plies, the second half-ply is formed of the same metal reinforcement elements as those of the first half-ply, said elements preferably being crossed with the elements of said first half-ply.
The working ply, as is known per se, is surmounted radially by a protective reinforcement composed of two plies of elastic metal reinforcement elements. One of said protective plies, preferably the radially inner ply, has an axial width greater than the greatest axial width of the working plies, whereas the width of the second protective ply has a width of a value which lies between the values of the widths of the working plies.